Search engines are indispensable tools for collecting and locating information. The typical search engines analyze users' input queries such as keywords or key phrases, locate, and return results. Search engines often use indexed information to locate relevant information that matches the users' needs. Many existing search engines are designed to classify a query by category and then locate relevant entries in the category. For example, the query for “Michael Jordan” may be classified in the category of sports and “Barack Obama” may be classified in the categories of news and/or politics.
Category-based query classification is useful for vertical search engines designed to search specific, already categorized information. It is, however, often unsuitable for identifying the user's intentions. For example, a typical category-based search engine can determine the query “Mobile Battery” is in the field of electronics; however, such a search engine often cannot easily figure out whether the user is looking for a mobile phone or for a battery, which results in poor relevance.